Strong magnetic fields are used in research and in diagnostic instruments. Magnet applications include Nuclear Magnetic Resonance (NMR), Fourier Transform Ion Cyclotron Resonance (FTICR), materials processing, gyrotrons, neutron diffraction, charge particle beam bending, and medical imaging MRI. Electrical winding magnets of various shapes can be used to produce such strong magnetic fields. For example, the shapes of such electrical winding magnets can range from generally circular to an increasingly racetrack shape. One such high field magnet is a superconducting magnet.
In addition to the magnetic field produced in the center region of such a magnet, which is generally the useful region for the intended purpose of the magnet, a magnetic field is also produced in a region located outside of the windings. This external field is often called the fringe field. The fringe field can be a source of interference with other processes being used in the vicinity of the magnet, including the proper operation of some instrumentation. Moreover, exposure to the fringe field can pose a recognized health hazard for humans. Therefore, methods have been developed for the reduction of the magnitude of the fringe field in the vicinity of high field magnets.
To reduce the fringe field, superconducting magnets have been constructed to include shields. Such shields may be passive or active. Passive shields consist of large quantities of low permeability material such as magnetic steel arranged or placed around the magnet. Active shields consist of a secondary electrical winding, or coil, surrounding the main coil of the magnet at some small but finite distance and operated with a polarity such as to cancel the field outside the combination of main magnet and shield coils.
In principle, there is no limit to the field reduction possible with the application of an active shield. The external field distribution of the main magnet may differ somewhat from that of the shield coil, but at some distance from the device the field distribution will correspond closely to an ideal dipole field. Thus, in an ideal configuration, the fringe field can be eliminated when the dipole moment of the shield coil is caused to be equal in magnitude and opposite in polarity to that of the main magnet.
In practice, however, there are limitations to the precision to which the active shield coil dipole moment is adjusted. The active shield coil is usually operated in series and in opposition with the main magnet, so the electrical energizing current is constrained to be the same. As a result, the dipole moment depends on the size of the shield coil and on the number of winding turns in the coil. Typical manufacturing variations in size or the balance of turns between the main magnet and the shield coil can lead to an imbalance in the dipole moment.
Moreover, the requirements on fringe field reduction from active shield coils are often such that the variations in the dipole moment of the shield coil, and the resulting small differences in the degree to which the fringe field is canceled, are of no practical importance. On the other hand, there are situations in which a very precise cancellation of the fringe field is required, or in which the cancellation must be adjusted by very small amounts from time to time. For these situations, a dipole shim coil may be effectively employed. The magnitude of the field required of the dipole shim coil is small in comparison with the magnitude of the field produced by the main magnet or the shield coil, and a correspondingly small number of windings are typically required. The provision for adjustment of the fringe field is possible because the dipole shim coil is a circuit that is independently energized from the main magnet and shield coil circuit. The coil is therefore properly referred to as a shim coil.
There are other shim coils that are often used together with superconducting magnets, for other purposes. But the dipole shim coil is distinguished by purpose and use. The purpose and required performance is also reflected in the design of the dipole shim coil as compared to shim coils intended for other purposes. The typical shim coils often associated with superconducting magnets are for the purpose of adjusting aspects of the central field including magnitude, variation of magnitude, and uniformity. Shim coils for the adjustment of uniformity are usually so-called gradient coils of various orders. Shim coils for the adjustment of the central field magnitude and time variations have a dipole moment, but the purpose, operation, and design of these coils of the magnet is to affect the central field. Such coils may also be placed on the outside of a main magnet windings due to space limitations on the inside of the magnet, or to benefit from the lower value of the ambient magnetic field on the outside of the magnet. Being a winding that surrounds a larger area than a shim coil inside the magnet, the shim coil on the outside of a magnet will have a larger dipole moment. But the purpose of the shim coils employed to date has been for the adjustment of the central field. In addition, a single coil does not obtain an independent adjustment of the central field and of the external fringe field simultaneously, and therefore the shim coils presently used do not suffice for the far fringe field adjustment of the present invention.
Accordingly, improvements in active shielding, which include allowing fine adjustment of the residual fringe field of an actively shielded superconducting magnet, are desired.